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  Harmony Search Algorithm to Predict Anomalous Zone Ahead of Tunnel Face

  During tunnelling work, emergence of unpredicted fault or anomalous zone leads to delay of construction work or increase of construction cost hampering construction ability. Predicting the anomalous zone ahead of tunnel face in advance can thus prevent time and economic loss. This raised the demand to develop a prediction technique that detects anomalous zones ahead of a tunnel face during tunnel excavation. So, there have been efforts in the tunnelling job sites to predict the ground condition ahead of a tunnel face, nowadays, using techniques such as Tunnel Seismic Prediction (TSP), pilot boring (drilling) and electromagnetic exploration etc. On top of that, additional efforts have been made to improve the accuracy and convenience of these techniques. Among the prediction techniques Rye et al (2008) attempted to detect the fault zone or weak zone ahead of tunnel face utilizing electrical resistivity by developing a Tunnel Electrical Resistivity Prospecting System. This system uses an inverse analysis algorithm to obtain the characteristics of an anomalous zone from the measured electrical properties of the ground by utilizing Monte Carlo method and Genetic Algorithm (GA)(Holland, 1975). Monte Carlo method usually requires a lot of calculation time to predict the characteristic values of an anomalous zone that exists ahead of a tunnel face, and can be applied only with enough and accurate information of the adjacent ground. On the other hand, if we are going to adopt the GA as an inverse analysis tool, the number of generation and the population size have to be decided empirically in advance to be input as initial values. It usually takes more than 30 minutes to perform inverse analysis using the GA. This study aims to improve the above mentioned problems by adopting Harmony Search (HS) algorithm; the effectiveness of the newly adopted algorithm was studied when performing an inverse analysis to obtain the characteristic values of an anomalous zone ahead of the tunnel face. The concept of Harmony Search was proposed by Geem et al (2001), and it was inspired by the principles of the harmony improvisation of musicians. The search for a perfect harmony is analogous to the procedure of finding the optimal solutions to engineering problems. The HS is a new concept of a heuristic algorithm that is known to show a good inverse analysis performance. This study derived the relationship correlating the electrical resistance of the ground (rock mass) with the four characteristic values (location, thickness, permittivity ratio and conductivity) of the anomalous zone based on Gauss ’ and Ohm ’ s laws when an anomalous zone exists in a rock mass. Inverse analysis using the HS algorithm has been developed for predicting four characteristic values from the measured electrical resistance

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